Satellite systems intended for broadcasting towards mobile terrestrial receivers generally operate in the so called L band covering the range of 1 to 2 GHz or in the so called S band which covers the range of 2 to 3 GHz. It turns out that in both these bands, the available bandwidth is limited due to limited frequency spectrum. Moreover these systems usually require the use of terrestrial repeaters in order to overcome effects of degradation and fading in the transmission produced by multipath effects and/or blockage of the line of sight from the satellite. For example in the urban areas where tall buildings exit the blockage effect can become more considerable.
Therefore, use is made of terrestrial repeaters that are usually installed at locations with high visibility towards the satellite. These repeaters are capable of receiving the satellite signal and, after certain internal processing, of re-transmitting the same signal within respective areas of coverage where the users can receive said signal with a desirable quality.
As it is known in the related art, the frequency bands used by said repeaters are different from those used by the satellites and they generally operate within frequency ranges where also spectrum resources are scarce.
In addition, due to the very nature of the multimedia broadcasting, large amounts of programs are usually needed to be transmitted to the users which in turn require relatively large system capacities. This factor is even more considerable in the European systems where multi-lingual multimedia applications are required, thus giving rise to a need for still higher capacities. It is to be noted that requiring larger system capacities in practical terms is to be interpreted as additional need for frequency resources which in the case of usage of terrestrial repeaters could result in the need for at least twice as much of frequency resources as that of a standalone satellite.
However, as it has already been mentioned above that the frequency resources are in practice limited, the direct consequence of a trade off in this respect would thus be the loss of system capacity.
Moreover, the currently existing satellite mobile broadcasting systems are usually single beam satellite systems. The satellite in such a system broadcasts time division multiplexing (TDM) bit streams under quadrature phase shift keyed (QPSK) modulation. The signal is then repeated by the above mentioned repeaters, for example in urban areas, using—for example—an orthogonal frequency division multiplexing (OFDM) waveform in a separate band.
DBS systems frequently make use of time diversity. Time diversity is a feature used so as to enhance terrestrial reception of the satellite signal transmission. According to this feature, use is made of an additional satellite located at a suitably separated position from the first satellite and transmitting substantially the same signal as that of the first one but within a predetermined time delay as compared to the signal transmitted from the first satellite.
In resource usage terms, the direct consequence of using time diversity is that broadcasting one TDM bit stream which would normally fit in one frequency block would require more than three frequency blocks; namely, one frequency block of the first satellite, one further frequency block for the satellite delayed replica and more than one for terrestrial repetition which requires a more robust coding. Therefore, while time diversity helps enhance the reception capacity at the user terminal end, it requires a higher level of usage of the resources.
In order to overcome the above drawbacks in usage of system resources, it is desired to provide a hybrid multispot satellite broadcasting system for providing multimedia programming to mobile users with a relatively high operating capacity while said system is capable of making efficient use of system resources during the terrestrial reception and re-transmission of the downlink signal towards the user.